The vertical displacement of the sacrum during walking is proposed as an estimation of the overall biomechanical performance of walking, independent of cardiopulmonary factors. Vertical sacral displacement during walking was measured using an optoelectronic motion analysis system in 10 normal volunteers at variable speeds. Oxygen consumption was simultaneously measured. The actual vertical displacement of the sacrum, when correlated with body weight, reliably predicted oxygen consumption (r2 = 0.91). The relationship between vertical sacral displacement and oxygen consumption persisted when controlling for the variables, velocity, square of velocity, cadence or stride length, each known to co-vary with cardiopulmonary performance. A mathematic model designed to predict the vertical displacement of the sacrum, based on sacral height and average stride length, was tested. The actual vertical sacral displacement correlated with predicted displacement (r = 0.94). Unilateral immobilization of each subject's knee resulted in a greater average vertical sacral displacement during gait than predicted. Comparing measured with predicted vertical sacral displacement may provide a clinically useful and specific overall assessment of biomechanical gait performance.